1. Field of the Invention
The present invention relates generally to video signal processing apparatus and, more particularly to a video signal processing apparatus for use in video monitor apparatus, such as a television receiver or the like.
2. Description of the Related Art
In video monitor apparatus such as a television receiver or the like, it is frequently observed that, when a supply of a video signal to be displayed thereon is interrupted a picture screen is disturbed and the brightness of the picture screen is changed considerably. More specifically, when a reception channel of a television receiver, for example, is switched, a synchronization disorder and a fluctuation of the brightness of the picture screen or the like occur until the channel-selection or tuning operation is finished. During such tuning operation the picture screen becomes quite unstable. As a result, there arise the following disadvantages:
(1) A picture is difficult to see; PA1 (2) A circuit is stressed; PA1 (3) A superimposed video signal such as a display on the picture screen and a superimposed picture of a so-called picture-in-picture, become unstable so that the picture is difficult to see or its content cannot be discriminated. PA1 (1) A stress applied to the circuit by the synchronization disturbance cannot be alleviated; PA1 (2) Nothing is displayed on the picture screen and this condition cannot be distinguished from the condition that a video apparatus is turned off; PA1 (3) A variety of superimposed video signals cannot be seen; and PA1 (4) Operation conditions of the video apparatus cannot be understood.
Further, when there is not a video signal, a vertical oscillation becomes a free running oscillation and a free running frequency is generally set to be lower than a normal frequency. As a consequence, a vertical amplitude is extended and it is frequently observed that a superimposed video signal such as a display area provided on the upper and lower ends of the picture, that is a superimposed picture of a picture-in-picture system, is displayed on the outside of the picture screen and cannot be correctly displayed.
According to the conventional video signal processing apparatus, in order to eliminate the above-mentioned disadvantages, the picture screen is processed by a blanking process when an input video signal is switched, for example, so as to prevent a unstable picture from being displayed on the picture screen. However, this proposal has the following disadvantages: